A plurality of systems uses a battery implemented as a battery pack or battery array, including a plurality of battery cells connected in series with each other.
When such a battery cell is charged to a much higher voltage or a much lower voltage than the voltage within a rated charge range, it may be dangerous.
Further, imbalance in the charged state of battery cells is caused by various factors, and occurs during the manufacture of batteries or the charge or discharge of batteries. In particular, in the case of lithium ion cells, the manufacture of cells is strictly controlled within a company to minimize the differences between the capacities of the cells of a battery array. However, imbalance or inequality between cells may occur due to various factors, regardless of the states of the cells, in which balance or equality is maintained after the cells are initially manufactured.
The factors influencing the imbalance of cells may include, for example, the chemical reactions, impedances and self-discharge rates of respective cells, reduction of the capacities of the cells, variation in the operating temperatures of the cells, and other types of variation between the cells.
Inconsistency in the temperature of cells is an important factor responsible for causing imbalance in cells. For example, “self-discharge” is caused in a battery cell, and is a function of a battery temperature. A battery having a high temperature typically has a self-discharge rate higher than that of a battery having a low temperature. As a result, the battery having a high temperature exhibits a lower charged state than the battery having a low temperature, with the passage of time.
Imbalance is a very series problem in the charged state of a battery. For example, this problem may typically occur in electric vehicles, and the capability of a battery to supply energy is limited by the battery cell having the lowest charged state.
If this battery cell is consumed, other battery cells lose the ability to continue to supply energy. This is the same even if the other battery cells still have the ability to supply power. Therefore, an imbalance in the charged state of battery cells reduces the power supply capability of the battery.
Of course, the above description does not mean that when one or more battery cells are consumed the supply of power by the remaining battery cells is completely impossible. However, it means that, only in the case of series connection, even if one or more battery cells are fully consumed, the battery can be continuously used as long as charge remains in the remaining battery cells, but, in that case, voltage having a reversed polarity is generated in the battery cell which has been fully discharged, and, as a result, the battery cell may be in danger of explosion due to the overheating thereof, or due to the generation of gas, and thus the battery loses power supply capability.
Various methods of correcting imbalance in the charged states of battery cells have been proposed, and one of the methods is shown in FIG. 1.
FIG. 1 is a diagram showing a conventional centralized charge equalization apparatus.
Referring to FIG. 1, the conventional centralized charge equalization apparatus includes a transformer T, N semiconductor switching devices D1 to Dn, a control switch SW, and a voltage detection and drive signal generation unit 10.
The transformer T is constructed such that it includes a single primary winding and N secondary windings, the N secondary windings are wound around a single common core, and the primary winding and the secondary windings have different polarities. In other words, a dot formed on the primary winding and dots formed on the secondary windings are placed on different sides. The secondary windings of the transformer T have the same number of turns, and the turns ratio of the primary winding to the secondary windings is N1:N2.
The semiconductor switching devices D1 to Dn are connected between the first ends of the secondary windings and the cathodes (+) of the batteries B1 to Bn, and are configured to rectify energy that is supplied from the secondary windings to the batteries B1 to Bn.
The control switch SW is connected in series with the primary winding, and is configured to form a closed loop in response to a drive signal provided by the voltage detection and drive signal generation unit 10.
The voltage detection and drive signal generation unit 10 detects the voltages of respective series-connected batteries B1 to Bn, compares the detected voltages with a reference voltage, and generates a drive signal required to discharge batteries charged to voltages greater than the reference voltage, that is, overcharged batteries.
A charge equalization method performed by such a conventional centralized charge equalization apparatus is described below.
First, the voltage detection and drive signal generation unit 10 detects the voltages of N series-connected batteries B1 to Bn.
Thereafter, the voltage detection and drive signal generation unit 10 turns on the control switch SW when the voltage detected from any one of the N series-connected batteries B1 to Bn is greater than the reference voltage.
Accordingly, energy supplied by the N series-connected batteries B1 to Bn is converted into magnetic energy and is stored in the primary winding of the transformer T.
Thereafter, when the voltage detection and drive signal generation unit 10 turns off the control switch SW, the magnetic energy, stored in the primary winding of the transformer T, is converted into a charge, and thus the N series-connected batteries B1 to Bn are charged with the charge through the secondary windings and the semiconductor switching devices D1 to Dn.
In this case, when the control switch SW is turned off, a greater amount of charge moves to a battery having a relatively low voltage through the secondary windings wound around the common core of the transformer T, thus realizing charge equalization.
However, in the conventional centralized charge equalization apparatus, since a number of secondary windings corresponding to the number of batteries is wound around a single common core, an increasing number of secondary windings corresponding to the increasing number of batteries must be wound around the single common core when the number of series-connected batteries increases. Accordingly, there are problems in that it is difficult to manufacture the secondary windings of the transformer T, and the turns ratio of the primary winding to the secondary windings of the transformer T increases in proportion to the number of series-connected batteries, thus making it difficult to manufacture the primary winding as the number of batteries increases.
Further, the conventional centralized charge equalization apparatus is problematic in that, since a number of secondary windings corresponding to the number of batteries is wound around a single common core, the flow of charge into batteries cannot be individually controlled depending on the charged states of the series-connected batteries, and overcurrent cannot be prevented from flowing into batteries that are currently being charged.
Accordingly, there is a problem in that some batteries may be overcharged or overdischarged when the charge equalization of series-connected batteries is performed.